[en] We performed a high statistics numerical simulation of the three state Potts model in presence of an external field that penalizes one of the possible states. The value of the thermal critical exponent, that we have evaluated by means of the finite size real space renormalization group (FSRSRG) technique, indicates that such a model has a non-trivial critical behaviour. For validation purposes results concerning both (standard) Potts and Ising models are also presented. (orig.)

[en] We compare three updating methods (Metropolis, heat bath and overrelaxed) for pure lattice quantum chromodynamics, by directly measuring their efficiency in a typical mass calculation. (orig.)

[en] We study the 3d 3-state Potts model defined on a diamond lattice (with a coordination number 4). By using finite-size scaling techniques we establish the first-order nature of the phase transition. We study local observables, tunneling events and the correlation length of the system, and we clarify the details of the pseudo-critical behavior which the system undergoes close to the transition region. (orig.)

[en] We have submitted to a close scrutiny the genesis of the glassy (or better the fully incommensurate) state in arrays of superconducting junctions at zero temperature. From our numerical simulations, using the XY hamiltonian, it comes out that frustration by itself is a necessary but not a sufficient condition. The other needed element is the disorder. However only the positional disorder is effective in the formation of a truly incommensurate state. The oscillation damping in the ground state energy and in the magnetic moment, as a function of the frustration, is not linear. The critical frustration (characteristic of the short range ''order-disorder'' crossover) decreases almost linearly with the root mean square of plaquette area, but not in a ''universal'' manner. (orig.)

[en] We perform a renormalization group study of the first order phase transition of the two dimensional Z(10) Potts model for which an ambiguous determination of the critical temperature was claimed. Allowing the system to flip between the two different phases in the large-volume limit near the critical point one can determine the critical temperature without ambiguity. (orig.)

[en] A general method for the study of the complex zeros of the partition functions for lattice theories with fermions is applied to compact U(1) with dynamical fermions. In particular, we obtain the zeros of the partition function on the complex quark mass plane. On a small 4⁴ lattice we study in a systematic way the dependence of the zeros on the theory's parameters, on lattice artefacts and on the various tuning parameters we have implemented. (orig.)

[en] We present a general method for the study of the complex zeros of the partition function for lattice theories with dynamical fermions. In particular, we consider the zeros of the partition function on the complex quark mass plane. Preliminary results are presented for lattice QCD and for compact QED. (orig.)

[en] We make a renormalisation group study with the finite size real space renormalization group (FSRSRG) technique of the order of the transition in the three dimensional three state Potts model with nearest neighbor and next to nearest neighbour couplings. Our results provide evidence that the order-disorder transition from a high temperature phase to a low temperature phase is discontinuous not only in the region characterized by both couplings being positive but also when the next to nearest coupling is appreciably antiferromagnetic. In both cases the correlation length exponent, previously not known, converges to the value expected for a first order first transition but only for large volumes, especially in the antiferromagnetic case. (orig.)

[en] We study the possibility of avoiding the fermion doubling problem by using a random coupling. We use numerical simulations in order to study the theory in the strong disorder region. We find a sharp crossover as a function of the strength of the disorder. For weak quenched disorder we find that the species doubling survives, while for strong quenched disorder only with a particular choice of the random term (antihermitian) it is possible to get a theory that seems to avoid fermion doubling. (orig.)

[en] We perform a numerical study of the Potts model q=3 in three dimensions with nearest neighbour and next to nearest neighbour couplings by means of the finite-size renormalization group method. The analysis of the magnetic critical exponents is complementary to the one of the thermal critical exponent already presented by us and confirms our conclusions that the transition from the disordered phase to the low-temperature ordered phase is first order. (orig.)